1. Field of the Invention
The present invention relates to a cellular mobile telecommunications system, more particularly to a call hand-off system for a cellular mobile telecommunications system.
2. Description of the Background Art
As is well known in the art, a cellular mobile telecommunications system includes base stations, each of which covers a zone consisting of a plurality of cells. Mobile stations can be connected to one of the base stations via radio link. A call which is connected between a base station and a mobile station moving toward another base station adjacent to the earlier-mentioned base station must be handed over to the other base station, toward which the mobile station is moving. The call hand-off is dealt with call hand-off sequences between the two base stations under the control of a base station control office. The call hand-off is one of the characteristic features of the cellular mobile telecommunications system, regardless of the radio link scheme, such as analog or digital transmission system, or FDMA (frequency-division multiple access), TDMA (time-division multiple access) or CDMA (code-division multiple access) system.
With the CDMA cellular telecommunications system standardized by the U.S. regulation, TIA/EIA/IS-95, for example, the off-sets of the pilot PN (pseudo-noise) sequence are different from each other to efficiently or commonly use the radio frequency channels. In correspondence therewith, each mobile station includes a plurality of code demodulators common to a single radio receiver system so as to establish a seamless diversity receiving.
The CDMA cellular telecommunications system confronts the difficulty in call hand-off. Specifically, by the common hand-off sequence, a mobile station connected to a base station will be handed off to another base station when the mobile station has completed the hand-off sequence. This means that the common hand-off sequence is defined under the requirement that the hand-off control must be completed between a mobile station and a base station which has been connected to the mobile station until then.
More specifically, a mobile station send a hand-off request to a base station to which it is connected is receiving radio waves transmitted from the base station until it receives a hand-off command in response from the base station. If the mobile station moves, for example, and receives stronger radio waves emitted from another base station than the radio waves transmitted from the former base station, then there is interference, in the signals transmitted form the former base station to the mobile station caused by the stronger radio waves. Due to the interference of the signals, the mobile station is unable to decode signals from the former base station. With such a common hand-off control sequence as mentioned above, the call connection of the mobile station may be released from the former base station before completing the hand-off sequence. That phenomenon may take place more significantly in the band of 1.9 gigahertz (GHz) than 800 megahertz (MHz) because the diffraction loss in the band of 1.9 GHz is larger than in 800 MHz. The more sectors of a base station, the narrower the directivity of the antenna thereof, and thus the more significantly the aforementioned phenomenon may occur.